1. Field of the Invention
Aspects of the present invention relate to servo control in an optical disc drive, and more particularly, to a gap servo control method and apparatus in a near-field optical disc drive.
2. Description of the Related Art
Recently, near-field optical disc drives that achieve a large disc capacity and a high data transfer rate (DTR) have been suggested. Such a near-field optical disc drive is called a near-field recording and reproducing system. A near-field optical disc drive is a device that records and reproduces data to and from a disc by using light in a near-field in which diffraction of the light does not occur.
Such a near-field optical disc drive records and reproduces a large amount of information by recording and reproducing an information cell within hundredths of an angstrom (Å) unit using near-field optical technology and ultra microelectromechanical system (MEMS) technology. That is, by using a near-field optical disc drive, data of more than 20 Gigabytes, which is the amount of image data for a high definition television (HDTV) class that can be recorded with an image quality of MPEG-2 for more than 2 hours, can be recorded and reproduced to and from one surface of a disc having a 3-cm diameter; thus, a large amount of data can be recorded and reproduced from a microminiaturized disc.
Such a near-field optical disc drive includes a pickup having nanoscale precision and a gap servo control apparatus to control the pickup. The pickup is called an optical head unit. The pickup includes an objective lens and a solid immersion lens (SIL). The pickup concentrates light on a disc by maintaining a focus distance within tens of nm from the disc so that an optical spot concentrated on the disc becomes microsized. Thus, in recording, the near-field optical disc drive can record a large amount of information to the disc. In reproduction, like the recording, the near-field optical disc drive reproduces information by the pick up receiving reflective light while maintaining a focus distance within tens of nm from the disc and performing optoelectric conversion of the received reflective light.
The gap servo control apparatus controls the pickup so that the pickup can constantly maintain the distance within the tenths of a nm gap with the disc surface. In order to constantly maintain the tenths of a nm gap, the gap servo control apparatus must control the pickup to react to disturbances of the disc in the near-field. In order to control the pickup to react to disturbances of the disc, the gap servo control apparatus may increase a loop gain of a feedback controller.
However, when the gap servo control apparatus increases the loop gain of the feedback controller, a servo bandwidth increases; and therefore, the gap servo control apparatus may be sensitive to disc scratches, contamination, and electrical noise, thereby increasing the occurrence of an overshoot during pull-in of the pickup. Since the gap that must be maintained between the pickup and the disc surface is very small, such as tenths of a nm, if an overshoot occurs when an open loop state is changed to a closed loop state in the pull-in of the pickup, the possibility of a collision of the SIL and the disc is very high. Thus, since the conventional gap servo control apparatus is limited to an increase in feedback loop gain, it is difficult to ensure a sufficient disturbance cancellation capability.